The present invention relates to a charged particle beam apparatus for obtaining an image displayed by irradiating a charged particle beam on a specimen and detecting a generation signal inclusively indicative of secondary electrons generated from the specimen.
A scanning electron microscope (hereinafter simply referred to as SEM) is available as the charged particle beam apparatus for obtaining an image by irradiating a charged particle beam on a specimen and detecting a signal secondarily generated from the specimen. Especially, in a semiconductor wafer inspection apparatus based on the SEM, a semiconductor wafer surface having a large area is observed and therefore an SEM image of high resolution needs to be acquired while maintaining a high throughput. For the sake of acquisition of a high-resolution SEM image, it is necessary that adjustment items of focus, image magnification and image rotation angle as well as dependency of these factors upon the height of a specimen be adjusted correctly. The items, however, change with an optical condition for irradiation of an electron beam on the specimen. The optical condition includes, for example, accelerating voltage of electron beam, convergence half-angle, electron beam current value and voltages applied to individual electrodes. On the other hand, with a view to maintaining a high throughput, there is no room for time to adjust the aforementioned adjustment items at each observation and it is desirable to precedently adjust and set an optical condition to be used, that is, to complete condition setting in advance. Two methods to this end have hitherto been known as will be described below.
In an image acquisition method according to prior art JP-A-2001-6588 (Patent Document 1), an SEM image is actually acquired under an optical condition desired to be set and adjustment items of focus, image magnification, image rotation angle and dependency of these factors upon specimen height are acquired and memorized when the sharpest image can be obtained. This prior art method introduced in the Patent Document 1 is obvious in setting an optical condition in the SEM.
In a function method according to prior art JP-A-2001-93831 (Patent Document 2) or U.S. Pat. No. 6,630,681, images are obtained to set several standard optical conditions. A function (in the form of either a simplified expression or a series expansion expression together with expansion coefficient table) is determined through which adjustment values under another optical condition can be calculated by interpolating adjustment values under a standard optical condition. When an optical condition desired to be set takes place, this condition is set by using the function.
In the image acquisition method as above, condition setting can be accomplished with high accuracy because setting is done while watching an actual SEM image. In the function method, on the other hand, any actual image need not be acquired under each condition setting and therefore the condition setting can be done easily within a short period of time.
But, in the image acquisition method, there needs acquisition of actual images corresponding in number to adjustment items each time that an optical condition is set and the more the adjustment items including preparation of specimens for adjustment and focus adjustment, the more the time is consumed. Further, since the user engaged in a charged particle beam apparatus employed frequently in recent years for observation, inspection and metrology of semiconductor wafer and the like is highly specialized in its operation, a worker specialized in condition setting is specially required.
On the other hand, in the function method, determination of a function must be done by trial and error in many cases each time an apparatus is designed and for example, in the case of new design of apparatus, much time is required for introducing a function. Further, even in an apparatus of the same type, adjustment values are uneven or dispersive among a plurality of products of the apparatus and the function needs to be added with parameters for adjusting itself in compliance with individual products of the apparatus. Furthermore, the parameters are empirical which fail to directly reflect causes of actual unevenness (unevenness in working, assembling and adjustment of parts of the apparatus) and hence, there is a possibility that a difference will occur between a setting value based on the function method and a true setting value. This limits accuracies of interpolation through the function. Accordingly, for ultimate settlement of adjustment values, comparison/confirmation/adjustment is sometimes needed in relation to an actual image and operation becomes complicated and time-consuming.
As an example of correcting an optical condition without resort to the two prior art methods, there is available a method according to which a means for calculating a trajectory of a charged particle beam is carried in a charged particle beam apparatus. Specifically, according to a charged particle beam exposure apparatus disclosed in Patent Document 2, an optical condition affected by space charge effect can be corrected by the charged particle beam trajectory calculation means mounted to the exposure apparatus but when setting of optical conditions in the wide range is required, a method is desired which can obtain highly accurately not only correction terms attributable to the space charge effect but also all of the aforementioned adjustment items through the use of the trajectory calculation without resort to acquisition of actual images.